This invention relates to a method and apparatus for mixing a resin and a catalyst and, more particularly, to an apparatus for efficiently mixing a high viscosity heavily filled resin with a catalyst and pressurized air wherein the catalyst and pressurized air are mixed before introduction to the resin.
Resins have numerous uses including, but not limited to the construction of swimming pools, the exterior coating of buildings, the protective interior coating of tanks, as well as the protective coating of secondary containment walls. A resin such as polyester is typically applied to a surface with a catalyst such as methyl-ethyl-ketone peroxide. Catalysts allow the resin to polymerize and cure. The present state of the art methods of resin application involve the spraying of the resin and methyl-ethyl-ketone peroxide onto a particular surface with a spray gun. A variety of spray guns are known in the art.
An internal mix gun is often used when solvent emissions are a problem, because internal mixing limits the amount of atomized material and catalyst exiting the gun. Internal mix guns generally have three feed lines, a resin line and a catalyst line which feed into a manifold, and an air line. The resin and catalyst are typically mixed in the manifold. After mixing, the resin and catalyst are expelled from the gun in confluence through a nozzle or similar orifice with pressurized air from the air line. The pressurized air supplies sufficient pressure so that the resin and catalyst are sheared and atomized as they are expelled from the gun. A major drawback of this type of gun is that during a spraying operation, catalyzed resin often backs up into and catalyzes within the air supply. Catalyzed resin in the air supply leads to costly and time-consuming down time while the spraying operation is shut down and the air supply is cleared of any obstructions. Standard check valves are rarely effective as they quickly become hardened shut with catalyzed resin or the internal workings of the check valve become frozen with catalyzed resin. Yet another problem with this type of gun is that a portion of the catalyst supply line extends beyond the on/off valve (i.e. between the on/off valve and the manifold) so that when the device is turned off, some catalyst drains from the end of the supply line into the manifold thereby wasting catalyst.
A second type of gun typically used is an external mix gun. In an external mix gun, the resin and catalyst are atomized and expelled separately and directed toward one another. The resin and catalyst combine in the air shortly before contacting the article being treated. A major drawback of the external mix gun is the incomplete mixing of resin and catalyst, which often leads to patches of incompletely catalyzed resin appearing on the finished article. Such portions of uncatalyzed resin can produce points of weakness or blisters on the surface of the finished article.
A more important problem with external mix guns is the exterior atomization of the catalyst. Because of the incomplete mixing of the catalyst with the resin, much of the atomized catalyst disperses into the atmosphere and, more particularly, in the immediate work environment where the application is taking place. Concern over the safety of workers breathing catalyst contaminated air has led to numerous restrictions on the use of external mix guns. Such guns have even been completely banned in at least one state.
Yet another type of gun is disclosed in U.S. Pat. Nos. 5,388,767, 5,388,768, and 5,388,763. In the device disclosed in these patents the resin and catalyst are not mixed in a manifold nor are they mixed after they are expelled. Rather, in these devices, the resin and catalyst are separately introduced into a mixing tube where they mix as they migrate toward the spray tip of the mixing tube. Separately combined with the mixing tube is a pressurized air stream which introduces pressurized air into the mixing tube. The pressurized air aids in mixing the catalyst and resin in the mixing tube and also aids in expelling the catalyst/resin mixture out of the end (spray tip) of the mixing tube. One problem with this design is that in order to prevent the resin from backing up into the catalyst supply line in the event of a clog in the mixing tube, it is necessary to introduce the catalyst into the mixing tube at the same pressure as the resin is introduced, which can approach 3000 pounds per square inch (psi) depending on the viscosity of the resin. It is undesirable to introduce catalyst at such high pressures since catalyst can often be corrosive and dangerous. If a catalyst line ruptured at high pressure, the catalyst could spray violently thereby potentially causing serious damage to life and property. Another problem with the design disclosed in these patents is that it can be difficult for the thick resin to thoroughly mix with the watery catalyst as the catalyst and resin migrate through the mixing tube. In fact, the relatively high viscosity catalyst often creates its own path as it travels through the mixing tube instead of completely mixing with the resin, thereby resulting in incomplete mixing of the resin and catalyst.
The difficulties encountered in the prior art discussed herein above are substantially eliminated by the present invention.